Lithographic apparatus

ABSTRACT

A lithographic apparatus including a substrate table position measurement system and a projection system position measurement system to measure a position of the substrate table and the projection system, respectively. The substrate table position measurement system includes a substrate table reference element mounted on the substrate table and a first sensor head. The substrate table reference element extends in a measurement plane substantially parallel to the holding plane of a substrate on substrate table. The holding plane is arranged at one side of the measurement plane and the first sensor head is arranged at an opposite side of the measurement plane. The projection system position measurement system includes one or more projection system reference elements and a sensor assembly. The sensor head and the sensor assembly or the associated projection system measurement elements are mounted on a sensor frame.

This application is a continuation of U.S. application Ser. No.13/414,352 filed on Mar. 7, 2012, which claims priority and benefitunder 35 U.S.C. §119(e) to U.S. Provisional Patent Application No.61/450,929, filed on Mar. 9, 2011, the contents of which areincorporated herein in their entirety by reference.

FIELD

The present invention relates to a lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

WO 2010/032878 discloses a lithographic apparatus comprising a sensorhead configured to determine the position of a substrate table of thelithographic apparatus. The sensor head is arranged on a sensor armextending under the substrate table. The sensor arm is rigidly mountedon the metrology frame of the lithographic apparatus. The substratetable of the lithographic apparatus comprises a holding device to hold asubstrate in a holding plane. The substrate table further comprises agrid plate extending in a measurement plane parallel to the holdingplane. The grid plate is arranged below the holding device, such thatthe sensor head arranged on the sensor arm can cooperate with the gridplate to measure a position of the substrate table.

A drawback of the position measurement system of WO 2010/032878 is thatthe position measurement system may be sensitive for dynamic movementsand thermal influences, which may result in inaccurate positionmeasurement. Inaccurate position measurement may result in exposureerrors such as focus and overlay errors, and is therefore undesirable.

SUMMARY

It is desirable to provide a lithographic apparatus comprising aposition measurement system for accurate position measurement of thesubstrate table.

According to an embodiment of the invention, there is provided alithographic apparatus comprising: an illumination system configured tocondition a radiation beam; a support constructed to support apatterning device, the patterning device being capable of imparting theradiation beam with a pattern in its cross-section to form a patternedradiation beam; a substrate table comprising a holding device to hold asubstrate in a holding plane; a projection system configured to project,when the substrate table is positioned in an exposure area, thepatterned radiation beam onto a target portion of the substrate, and asubstantially vibration isolated frame supporting the projection system,wherein the lithographic apparatus comprises a substrate table positionmeasurement system to measure a position of the substrate table, whereinthe substrate table position measurement system comprises a substratetable reference element arranged on the substrate table, and a firstsensor head to determine a position of the first sensor head withrespect to the substrate table reference element, wherein the substratetable reference element extends in a measurement plane substantiallyparallel to the holding plane, and wherein the holding plane is arrangedat one side of the measurement plane and the first sensor head isarranged, when the substrate table is in the exposure position, at anopposite side of the measurement plane, wherein the lithographicapparatus comprises a sensor frame mounted on a sub-frame of thelithographic apparatus and wherein the first sensor head is mounted onthe sensor frame, wherein the lithographic apparatus comprises aprojection system position measurement system to measure a position ofthe projection system, comprising at least one projection systemreference element and a sensor assembly to determine a position of thesensor assembly with respect to the projection system referenceelements, and wherein one of the sensor assembly and the at least oneprojection system reference element is mounted on the projection system,and the other of the sensor assembly and the at least one projectionsystem reference element is mounted on the sensor frame.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts the sensor frame with sensor head and sensor assembly inmore detail according to an embodiment of the invention;

FIG. 3 depicts schematically a top view of the cross section A-A of FIG.2; and

FIG. 4 depicts a second sensor frame with sensor head and sensorassembly for the measurement area of the lithographic apparatusaccording to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or any other suitable radiation), a patterning devicesupport or support structure (e.g. a mask table) MT constructed tosupport a patterning device (e.g. a mask) MA and connected to a firstpositioning device PM configured to accurately position the patterningdevice in accordance with certain parameters. The apparatus alsoincludes a substrate table (e.g. a wafer table) WT or “substratesupport” constructed to hold a substrate (e.g. a resist-coated wafer) Wand connected to a second positioning device PW configured to accuratelyposition the substrate in accordance with certain parameters. Theapparatus further includes a projection system (e.g. a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g. including one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, todirect, shape, or control radiation.

The patterning device support supports holds the patterning device in amanner that depends on the orientation of the patterning device, thedesign of the lithographic apparatus, and other conditions, such as forexample whether or not the patterning device is held in a vacuumenvironment. The patterning device support can use mechanical, vacuum,electrostatic or other clamping techniques to hold the patterningdevice. The patterning device support may be a frame or a table, forexample, which may be fixed or movable as required. The patterningdevice support may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports”. In such “multiple stage”machines the additional tables or supports may be used in parallel, orpreparatory steps may be carried out on one or more tables or supportswhile one or more other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support MT (e.g., masktable), and is patterned by the patterning device. Having traversed thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioning device PW anda substrate table position measurement system comprising a sensor head 1mounted on a sensor frame 3 and a grid plate 4 mounted on the substratetable WT, the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioning device PM and another positionsensor (which is not explicitly depicted in FIG. 1) can be used toaccurately position the patterning device (e.g. mask) MA with respect tothe path of the radiation beam B, e.g. after mechanical retrieval from amask library, or during a scan. In general, movement of the patterningdevice support (e.g. mask table) may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioning device PM.Similarly, movement of the substrate table WT or “substrate support” maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the patterning device support (e.g. mask table)may be connected to a short-stroke actuator only, or may be fixed.Patterning device (e.g. mask) MA and substrate W may be aligned usingpatterning device alignment marks M1, M2 and substrate alignment marksP1, P2. Although the substrate alignment marks as illustrated occupydedicated target portions, they may be located in spaces between targetportions (these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the mask MA, themask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support MT or “mask support” andthe substrate table WT or “substrate support” are kept essentiallystationary, while an entire pattern imparted to the radiation beam isprojected onto a target portion C at one time (i.e. a single staticexposure). The substrate table WT or “substrate support” is then shiftedin the X and/or Y direction so that a different target portion C can beexposed. In step mode, the maximum size of the exposure field limits thesize of the target portion C imaged in a single static exposure.

2. In scan mode, the patterning device support MT or “mask support” andthe substrate table WT or “substrate support” are scanned synchronouslywhile a pattern imparted to the radiation beam is projected onto atarget portion C (i.e. a single dynamic exposure). The velocity anddirection of the substrate table WT or “substrate support” relative tothe patterning device support MT or “mask support” may be determined bythe (de-)magnification and image reversal characteristics of theprojection system PS. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion in a single dynamic exposure, whereas the length of the scanningmotion determines the height (in the scanning direction) of the targetportion.

3. In another mode, the patterning device support MT or “mask support”is kept essentially stationary holding a programmable patterning device,and the substrate table WT or “substrate support” is moved or scannedwhile a pattern imparted to the radiation beam is projected onto atarget portion C. In this mode, generally a pulsed radiation source isemployed and the programmable patterning device is updated as requiredafter each movement of the substrate table WT or “substrate support” orin between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIGS. 2 and 3 show the substrate table position measurement systemaccording to an embodiment of the invention.

The substrate table position measurement system comprises a sensor head1 which is arranged on a first arm 2 of a sensor frame 3. The sensorhead 1 is configured to cooperate with a substrate table referenceelement in the form of a grid plate 4 arranged at a bottom side of thesubstrate table WT. The substrate table WT comprises a holding device orholder 5, for example a vacuum clamp, to hold the substrate W in aholding plane. The grid plate 4 extends in a measurement planesubstantially parallel to the holding plane. Any other substrate tablereference element capable of cooperating with the sensor head 1 todetermine the position of the substrate table WT may also be applied.

The sensor head 1 is configured to determine the position of thesubstrate table WT in six degrees of freedom. Therefore, the sensor head1 comprises a first and a second encoder sensor to measure a position ina first direction in the measurement plane, for example the x-direction,and a third encoder sensor to measure a position in a second directionin the measurement plane substantially perpendicular to the firstdirection, for example the y-direction. The sensor head 1 comprisesthree interferometer sensors to measure a position in a directionsubstantially perpendicular to the measurement plane, for example thez-direction.

In the embodiment of FIGS. 2 and 3, the sensor head 1 comprises threemeasurement sensors 1 a, each combining one of the encoder sensors andone of the interferometer sensors. The distance between the individualmeasurement sensors 1 a may for instance be in the range of thedimensions of a die on the substrate W resulting in a sensor head 1 witha relative small measuring area to measure the position of the substratetable WT in six degrees of freedom.

In an embodiment, the sensor head 1 is arranged on the optical axis O ofthe lithographic apparatus. As a result, the sensor head 1 can measurethe position of the grid plate 4 at a location very close to thelocation where, during exposure, a projection beam B is projected on thesubstrate W supported on the substrate table WT. In this way theposition of the target location of the substrate, i.e. where theprojection beam B is projected on the substrate W, can be relativelyaccurately determined and used for position control of the substrate Won the substrate table WT. Also, position measurement at this locationresults in low sensitivity with respect to deformations in the substratestage.

Since the sensor head 1 is arranged at an opposite side of themeasurement plane with respect to the holding plane of the holdingdevice or holder 5, the sensor head 1, although arranged on the opticalaxis O, does not interfere with the projection of the patternedradiation beam on the substrate W.

In an embodiment, the sensor frame 3 is manufactured from lightweightmaterial with low thermal expansion coefficient, for instance Zerodur.Furthermore, the sensor frame 3 has a lightweight construction withrelative high stiffness.

The lowest resonance frequency of the sensor frame 3 is at least 400 Hz,and in an embodiment, at least 600 Hz. Such relative high level of thelowest resonance frequency is in this example obtained by constructingthe sensor frame 3 lightweight and relatively stiff.

The sensor frame 3 is mounted on the metrology frame MF. This metrologyframe 3 is a substantially vibration isolated frame, i.e. asubstantially stationary frame, to which the projection system PS ismounted. It is remarked that a substantially stationary frame may be anyframe which is passively or actively held in a substantially stationaryposition. The metrology frame 3 of the lithographic apparatus is mountedwith passive or active air-mounts on a base frame to filter any externaldisturbances such as vibrations in the factory floor. In this way theprojection system PS and the sensor frame 3 are held in a substantiallystationary position.

However, some movements may still be present in the metrology frame, forinstance bending modes of the metrology frame typically in a frequencyrange of 150-200 Hz. Other movements of the metrology frame may alsoinfluence the measurement quality of the substrate table positionmeasurement system.

To make the sensor frame 3 less sensitive to movements of the metrologyframe, the sensor frame 3 is kinematically mounted in six degrees offreedom on the metrology frame MF of the lithographic apparatus using astatically non-undetermined construction, i.e. the construction is notunder determined or over determined. In the shown embodiment astatically determined (i.e. non-under determined and non-overdetermined) leaf spring construction 6 is used to mount the sensor frame3 on the metrology frame MF.

Since the lowest resonance frequency of the sensor frame (>400 Hz) issubstantially higher than the typical resonance frequencies of themetrology frame (150-200 Hz), and the sensor frame 3 is isolated fromthe resonance vibrations of the metrology frame MF by the leaf springconstruction 6, the performance of the substrate table positionmeasurement system is substantially improved.

It is desirable for good image quality that focus and imaging errors aresmall. For controlling these focus and imaging errors, it is desirableto control the position of the substrate table with respect to theprojection system PS, preferably independent of movements of themetrology frame. For this reason, a sensor assembly 7 is mounted on asecond arm 8 of the sensor frame 3, so that the position of thesubstrate table WT and the projection system PS with respect to eachother can be accurately be determined. The sensor arm 8 is partlyannular to measure the position with respect to an outer rim of theprojection system PS. On the basis of these measurements the position ofthe projection system at the optical axis O can be determined.

The sensor assembly 7 is configured to determine the position of theprojection system PS in six degrees of freedom. The sensor assembly 7comprises a first position sensor 9 and a third position sensor 11 tomeasure a position of the projection system PS in a radial directionwith respect to the optical axis O of the projection system, and asecond position sensor 10 to measure a position of the projection systemPS in a tangential direction with respect to the optical axis O of theprojection system PS. The sensor assembly 7 further comprises threeposition sensors 12 to measure a position of the projection system PS ina direction substantially parallel to the optical axis O of theprojection system, i.e. in z-direction.

The position sensors 9, 10, 11 and 12, may for instance be encoder-typesensors, interferometers and/or capacitive sensors, and are configuredto determine a position with respect to reference elements 13, forinstance small mirror elements or grid plates, arranged on an outer rimof the bottom end of the projection system PS. In alternativeembodiments, any other suitable type of sensors and reference elementsmay also be used. The reference elements may for example also be formedby the projection system itself.

Since the sensor assembly 7 and the sensor head 1 are mounted on thesensor frame 3, the relative position of the substrate table WT withrespect to the projection system PS can be measured with high accuracy.

It is remarked that in an alternative embodiment the sensor assembly 7may be mounted on the projection system PS, and the projection systemreference elements 13 may be mounted on the sensor frame 3.

The quality of the position measurement may be influenced by resonancefrequencies in the sensor arm itself, position changes in the sensor arm3 due to sensor frame deformations caused by air pressure forcesresulting from movements of the substrate table WT, thermal effects inthe sensor frame 3, for instance caused by local heating, anddisturbances in the measurement beams due to thermal effects in the areabetween sensor head 1 and the gird plate 4.

The following measures may be taken to decrease or even take away theseeffects.

Although the lowest resonance frequencies of the sensor frame 3 aresubstantially higher than the lowest resonance frequencies of themetrology frame MF, these resonance frequencies of the sensor frame 3may still influence position measurement. To decrease the effect ofresonance frequencies of the sensor frame 3 itself, an active dampingdevice or active damper 15 is provided to damp movements of the sensorframe 3. This active damping device or active damper 15 may beconfigured as any damping device or damper 15 capable of suppressing themovements of the sensor frame 3. As an alternative, or in additionthereto, a passive damping device may be provided to damp movements ofthe sensor arm 3. Such a passive damping device may be a tuned massdamper for suppressing one or more resonance peaks of the sensor frame3. Alternatively or additionally, an eddy current based damper may beused.

The movement of the substrate table WT for instance during scanningmovements of the substrate table may cause pressure waves whichpropagate in the area where the sensor head 1 is provided. Also at otherlocations pressure waves may be present during the use of thelithographic apparatus which may act on the sensor frame 3, or partsthereof. To decrease the influence of the pressure waves one or moreshielding frames comprising shielding material at least partiallyenclosing the sensor frame 3 may be provided. In the exemplaryembodiment of FIGS. 2 and 3 a shielding frame 16 is provided whichencloses the sensor arm 2 and the sensor head 1. Additional shieldingframes may be provided for other parts of the sensor frame 3 and/or thesensor assembly 7.

Additionally or alternatively, the sensor frame 3 may have anaerodynamic design to minimize the influence of pressure waves.

In an embodiment, the one or more shielding frames 16 are not mounted onthe sensor frame and/or the metrology frame, since pressure wave forceswill be exerted on these shielding frames 16. The one or more shieldingframes 16 may for instance be mounted on a base frame of thelithographic apparatus, i.e. a non-vibration isolated frame of thelithographic apparatus.

The thermal conditions within the sensor frame 3 may also have aninfluence on the relative position between the sensor head 1 and thesensors of the sensor assembly 7. These relative positions should bekept constant to obtain a reliable position measurement of the substratetable WT with respect to the projection system PS. To control theeffects caused by thermal effects within the sensor frame 3, one or morethermal conditioning devices or thermal conditioners 17 may be providedto control thermal condition of the sensor frame 3. These thermalconditioning devices or thermal conditioners 17 may, for example,comprise water heating/cooling conduits, local heaters which may beelectric, or such. The thermal conditioners 17 may comprise temperaturecontrolled liquids and/or gasses. The thermal conditioners 17 maycomprise peltier-elements or heat-pipes to transport heat. The thermalconditioning devices or thermal conditioners 17 may be mounted on thesensor frame 3 or on any other suitable location, for example ashielding frame 16. A temperature sensor may be located in or on thesensor frame 3 to acquire an accurate temperature.

As explained above thermal conditions in the measurement beam area, i.e.between the sensor head 1 and the grid plate 4 may cause disturbances inthe measurement. One or more air conditioning devices or airconditioners 18 to provide a conditioned air flow in a measurement areaof measurement beams of the sensor head 1. Similarly air conditioningdevices 18 may be provided to condition the area wherein the measurementbeams of the position sensors 9, 10, 11, 12 of the sensor assembly run.The air conditioning devices 18 may be located on at least one of thesensor frame 3, the metrology frame MF and the base frame.

The one or more air conditioning devices 18 may be mounted on anysuitable location, for instance the shielding frame 16. It is remarkedthat when the measurement beam run through a conditioned environment,for instance a partially vacumized environment, the air conditioningdevices 18 may be used to condition this conditioned environment.

FIGS. 2 and 3 show the sensor frame 3 to measure the position of asubstrate table WT in the exposure area of the lithographic apparatus,i.e. where a patterned projection beam B is projected on the substrateW.

The sensor frame 3 of the invention may also be provided in themeasurement area of the lithographic apparatus. Before actual exposureof patterns on the substrate W, the surface of the substrate W isscanned by an alignment sensor and a focus sensor to measure the surfaceof the substrate W. This information is used during the exposure phaseto optimize the alignment and focus of the substrate surface withrespect to the projection system.

FIG. 4 shows the sensor frame 3 arranged in such measurement range. Thesame parts or parts having substantially the same function are providedwith the same reference numerals. It is remarked that in thelithographic apparatus, the measurement area and the exposure area maybe located close to each other, preferably adjacent to each other.

On the metrology frame MF, an alignment sensor 20 and a focus sensor 21are arranged to perform alignment and focus measurements. A secondsensor frame 3 is kinematically mounted on the metrology frame MF with astatically determined (i.e. non-under determined or non-over determined)leaf spring construction 6. The second sensor frame 3 comprises a firstsensor frame arm 2 and a second sensor frame arm 8. On the first sensorframe arm 2, a second sensor head 1 is mounted to determine a positionof the substrate table WT with respect to the sensor frame 3. On thesecond sensor frame arm 8 a second sensor assembly 7 is mounted todetermine positions of the alignment sensor 20 and the focus sensor 21with respect to the sensor frame 3. Reference elements 13 are mounted onthe alignment sensor 20 and the focus sensor 21 to cooperate with thesensor assembly 7.

With the second sensor frame 3, the second sensor head 1 and the secondsensor assembly 7, a reliable measurement of the relative positionbetween the substrate table WT and the alignment sensor 20 and the focussensor 21 can be obtained.

It is remarked that similar to the embodiment of FIGS. 2 and 3, thesensor frame 3 may be provided with further measures to optimize themeasurement performance, such as active or passive damping devices, oneor more shielding frames, one or more thermal conditioning devices forthe sensor frame 3, and one or more air conditioning devices tocondition the air in the area of the measurement beams.

In the embodiments described above, the substrate table referenceelement may be made of the same material as the substrate table, or asat least the part of the substrate table. This part may be the part thatexists between the substrate table reference element and the top surfaceof the substrate table that requires high position accuracy. This hasthe benefit that thermal deformation of the substrate table can bemeasured with the substrate table reference element. Also thermaldeformations that have horizontal gradients or which vary over time maybe measured. When the substrate table expands due to an increasedtemperature, the substrate table reference element expands in the sameway. Thermal deformations in a plane parallel to the holding plane canbe directly measured at the point of interest, i.e. at the exposure sliton the substrate. Thermal deformations in a direction perpendicular tothe holding plane may still exist, but are typically a factor 10 less ofimportance for the lithographic process. This allows the use ofmaterials different than materials with an extreme low thermal expansioncoefficient, CTE, such as Zerodur. Instead, materials with a higher CTE,and a higher Young modulus or higher thermal conductance may be used,such as SiSiC. In an embodiment, the substrate table and the substratetable reference element are one monolithic part.

Alternatively or in addition with the embodiments described above, thelithographic apparatus may be provided with an additional positionmeasurement system for measuring the position of at least part of thesubstrate table position measurement system relative to at least partthe projection system position measurement system. An example of anadditional position measurement system is an interferometer system tomeasure the position of the substrate position measurement systemrelative to the projection system position measurement system. Forexample, the interferometer system may measure the position of thesensor assembly 7 relative to the sensor head 1. Thermal deformation orvibrations of the sensor frame can be detected by the interferometersystem and may thus be compensated for. The interferometer system may bean external interferometer system in which the interferometer beams gooutside the sensor frame 3. The interferometer system may be an internalinterferometer system in which the beams go through the sensor frame 3.The sensor frame 3 may be made at least partly hollow or provided with atransparent inner part, so the beams may go through. The internalinterferometer system has the benefit that the beams can be shielded fordisturbances, such as pressure waves. The external interferometer systemmay be provided with additional shielding against thermal, optical orsonic disturbances. Alternatively or in additional to the interferometersystem, a spectral interference laser may be used.

Other examples of an additional position measurement system are anoptical encoder system, an magnetic encoder system and anon-interferometer system. Examples of non-interferometer systems aretriangulation, detection of an optical spot on at least one photodiode,or using CCD's.

A further example of an additional position measurement system is astrain-based measurement system. This system may comprise a strain-gaugelocated in or on the sensor frame 3.

In a further example of an additional position measurement system thesensor frame 3 may be provided with at least one optical fiber fortransporting light. Deformation of the sensor frame 3 can causedeformation of the fiber and as a result a change in a property of thelight through the fiber, such as polarization or intensity. This changecan be measured by a suitable sensor.

In the embodiments described above, a further position measurementsystem may be provided to measure at the top side of the substrate theposition of the substrate or substrate table in Z, Rx and/or Rz.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1-20. (canceled)
 21. A lithographic apparatus, comprising: a substratetable comprising a holding device to hold a substrate; a substrate tableposition measurement system comprising a sensor head and a substratetable reference element; and a further position measurement system,wherein the substrate table reference element is arranged at a bottomside of the substrate table, wherein the sensor head is configured tocooperate with the substrate table reference element to determine aposition of the substrate table, and wherein the further positionmeasurement system is configured to measure at a top side of thesubstrate the position of the substrate table.
 22. The lithographicapparatus of claim 21, comprising a substantially stationary frame and asensor frame, wherein the sensor head is arranged on the sensor frame,and wherein the sensor frame is mounted on the substantially stationaryframe.
 23. The lithographic apparatus of claim 22, comprising aprojection system configured to project a pattern imparted to aradiation beam by a patterning device onto a target portion of thesubstrate, wherein the projection system is mounted to the substantiallystationary frame.
 24. The lithographic apparatus of claim 22, comprisinga passive damping device configured to damp a movement of the sensorframe.
 25. The lithographic apparatus of claim 24, wherein the passivedamping device is a tuned mass damper for suppressing a resonance peakof the sensor frame.
 26. The lithographic apparatus of claim 22,comprising an active damper configured to damp a movement of the sensorframe.
 27. The lithographic apparatus of claim 22, wherein the sensorframe is kinematically mounted in six degrees of freedom to thesubstantially stationary frame.
 28. The lithographic apparatus of claim21, wherein the sensor head is arranged on an optical axis of thelithographic apparatus.
 29. The lithographic apparatus of claim 21,wherein the sensor head is configured to determine the position of thesubstrate table in six degrees of freedom.
 30. The lithographicapparatus of claim 21, wherein the substrate table reference element ismade of the same material as at least part of the substrate table. 31.The lithographic apparatus of claim 21, wherein the sensor frame ismanufactured from a lightweight material with a low thermal expansioncoefficient.
 32. The lithographic apparatus of claim 21, comprising ashielding frame comprising shielding material at least partly enclosingthe sensor frame.
 33. A lithographic apparatus, comprising: a substratetable comprising a holding device to hold a substrate in a holdingplane; a first measurement system comprising a sensor head and areference element, the sensor head and the reference element eacharranged at a position below the substrate when held by the holdingdevice and the reference element extending in a plane substantiallyparallel to the holding plane, the sensor head configured to cooperatewith the reference element to determine a position of the substratetable; and a second position measurement system configured to determinethe position of the substrate table at a top side of the substrate. 34.The lithographic apparatus of claim 33, wherein the position is along adirection substantially perpendicular to the holding plane.
 35. Thelithographic apparatus of claim 33, wherein the reference element isarranged within the substrate table.
 36. The lithographic apparatus ofclaim 33, comprising a third measurement system configured to measure arelative position of the substrate table and a projection systemconfigured to project a patterned beam of radiation onto the substrate.37. The lithographic apparatus of claim 33, wherein the sensor head isarranged on a sensor frame that is mounted on a substantially stationaryframe that receives a projection system of the lithographic apparatus.38. The lithographic apparatus of claim 33, wherein the sensor head isarranged at a position on an optical axis of the lithographic apparatus.39. The lithographic apparatus of claim 33, wherein the sensor head isconfigured to determine the position of the substrate table in sixdegrees of freedom.
 40. The lithographic apparatus of claim 33, whereinthe reference element is made of the same material as at least part ofthe substrate table.